1. Field of the Invention
The present invention relates to a method and apparatus for testing a microporous hollow fiber membrane module for the presence or absence of a defect. More particularly, this invention is directed to a method and apparatus wherein, in the presence of any defect, the two dimensional position thereof is detected to enable proper repair and mending.
2. Description of the Prior Art
Hollow fiber membrane modules are widely used for various purposes such as filtration of aqueous or gaseous fluid, ultrafiltration of particulate materials and dialysis of blood. Generally, a hollow fiber membrane module includes a bundle of microporous hollow fibers arranged in a tubular casing, as illustrated, for example, in Japanese Patent Kokoku Publication No. 2-14084. In one form, the fiber bundle is bent into the U-shape with the ends of respective fibers supported by a partition wall of potting resin bonded to the casing. The potting resin forming the partition wall serves to sealingly fill the interstices between fibers, with the inner channels of respective fibers being open onto the outer face of the partition wall. In another form, the straight fibers are used with their opposite ends bonded to separate partition walls.
Defect may occasionally be present for various reasons in the hollow fiber membrane modules as fabricated. For instance, during extrusion spinning of the hollow fibers, pinholes or fissure may inadvertently be formed in the microporous membrane forming the hollow fibers. Furthermore, the hollow fibers may be damaged during subsequent handling. In addition, voids and cavities formed across the partition wall during molding of potting resin as well as shortage of bonding between fibers and potting resin may result in short-paths of fluid circumventing the micropores of the hollow fibers.
Japanese Patent Kokoku Publication No. 2-14084 discloses a method of testing a hollow fiber membrane module for a defect wherein air carrying fine airborne particles is drawn across the module and is forwarded to a particle counter for counting the number of particles that have passed the module. The sensitivity of the particle counter is adjusted to count only those particles having a size larger than a predetermined level. If any particles are counted, it is judged that the module is defective.
While this method permits inspection of the modules for the presence or absence of a defect, repair of defective modules is impossible because the specific position of defect is not identified. Therefore, the defective modules must be discarded as a whole without repair even though only a single fiber is defective. This lowers the production yield and leads to a waste of materials and resources.
Japanese Patent Kokai Publication No. 53-134776 describes a method of detecting the position of a defective portion of a hollow fiber membrane module. The method includes supplying a pressurized gas to the unfiltered side of the module to cause the gas to leak through the defect. The rate of gas leakage through a defective portion will be higher than the flow rate across nondefective fibers so that a refraction anomaly is resulted at the defective portion. This refraction anomaly is visualized by an optical system according to the schlieren method and the location of the defective portion is identified. Once the position of the defect is determined, an adhesive may be applied to seal the defective portion as disclosed in Japanese Patent Kokai Publication No. 54-138874.
According to this method, however, it is often difficult to distinguish the refraction anomalies with reliability because, due to the fluctuation in the flow resistance through respective hollow fibers, the velocity of gas flowing out of the inner channels of respective fibers varies considerably from fiber to fiber. Moreover, it is extremely difficult to find out the refraction anomalies resulting from microscopic defects which are of a size in the order of micrometers or less and which would nevertheless affect the quality of the module.